1. Technical Field
The present invention relates to a control device for a machine tool, and in particular, to control in the event of power failure.
2. Background Art
In a machine tool, a workpiece or a tool is controlled to machine the workpiece in a predetermined shape. For example, in a lathe or the like, a spindle on which the workpiece is mounted is rotationally driven by a spindle motor and a feed shaft for moving the workpiece (spindle) or the tool is linearly driven or rotationally driven by a servo motor through a ball screw, a gear, or the like. On the other hand, in a machining center or the like, a spindle on which the tool is mounted is rotationally driven by a spindle motor, and a feed shaft for moving the workpiece or the tool (spindle) is linearly driven or rotationally driven by a servo motor through a ball screw, a gear, or the like.
FIG. 3 is a block diagram showing one example control device of related art for control of the machine tool in the event of power failure. An alternate current (AC) input power supply 1 is input to a power circuit 2. The power circuit 2 converts the input AC voltage to a direct current (DC) voltage, and supplies the DC voltage to servo motor amplifiers 3 and 4 and an amplifier 5 for a spindle motor. A numerical control device 6 inputs instruction values to the amplifiers 3 and 4 for the servo motors and the amplifier 5 for the spindle motor, to machine the workpiece into a predetermined shape. The amplifiers 3 and 4 for the servo motors and the amplifier 5 for the spindle motor drive servo motors 7 and 8 and a spindle motor 9 according to the instruction values from the numerical control device 6. A power failure detecting unit 10 constantly monitors the input power supply 1, and when the power failure is detected, because the power supply precision to the motor is reduced due to reduction of the DC voltage and the workpiece and the tool cannot be precisely controlled, a suspension of electricity application is instructed to the amplifiers 3 and 4 for the servo motors and the amplifier 5 for the spindle motor so that the driving of the servo motors 7 and 8 and the spindle motor 9 can be interrupted.
When power failure occurs during machining of the workpiece, because the motive power is cut off, the servo motors 7 and 8 stop with a dynamic brake regardless of the instruction value from the numerical control device 6, and the spindle motor 9 stops in a free-run state. The servo motors 7 and 8 and the spindle motor 9 are operated in a non-controlled state until the motors stop, and thus there is a problem in that the workpiece or the tool is damaged.
As a technique for avoiding damaging of the workpiece or the tool, a method is known in which the spindle motor is decelerated and stopped under a controlled state in the event of the power failure, and the regenerative energy obtained during the deceleration is used to drive the servo motor (for example, refer to Japanese Patent No. 3001377).
In addition, as another technique for avoiding damaging of the workpiece or the tool, a method is known in which the deceleration of the spindle motor is controlled such that the regenerative energy obtained during deceleration of the spindle motor does not exceed the energy necessary for driving the servo motor (for example, refer to Japanese Patent No. 3369346).
With the above-described related art, the workpiece and the tool can be retracted to an area where there is no interference, and damaging of the workpiece or the tool can be avoided. However, with the technique of JP 3001377, a resistive electricity discharge unit for consuming redundant energy which is not consumed by the driving of the servo motor must be provided. On the other hand, in the technique of JP 3369346, the time for retracting the workpiece and the tool to the non-interfering area is elongated, and thus an uninterrupted power supply device must be provided for the numerical control device so that the control can be continued in the event of the power failure. In other words, in both techniques, there had been a problem in that the cost becomes higher because the resistive electricity discharge unit or the uninterrupted power supply device, which are unnecessary for normal control, must be provided.
An advantage of the present invention is that damaging of the workpiece or the tool is avoided by retracting the workpiece and the tool to the non-interfering area without adding the resistive electricity discharge unit or the uninterrupted power supply device.